How to redirect and skip sign in on one domain after signed up on another?

I have a domain and a subdomain, lets say root.com and cms.root.com

root.com and cms.root.com have different database.

I want to create a signup flow on root.com and after successful signup on root.com redirect user to cms.root.com skipping the signin of cms.root.com (cms.root.com already has a signin/signup flow)

how can I achieve this, and what would be the best way?